Thermistor mounts



gpril 19, 1960 s, HOPPER 2,933,705

\ THERMISTOR MOUNTS Filed 001:. 25, 1955 IN VENTOR SAMUEL HOPF E R ATTORNEY Uite States Patent 2,933,705 THERMISTOR MOUNTS Samuel Hopfer, Brooklyn, N.Y., assignor to Polytechnic Research & Development Co., Inc., Brooklyn, N.Y., a corporation of New York Application October 25, 1955, Serial No. 542,655

4 Claims. (Cl. 333-98) This invention relates to devices for the measurement of microwave energy transmitted in waveguides. Speciflcally, the invention is concerned with a waveguide mounting for a thermistor head for the measurement of microwave energy.

A broad object of the invention is to devise a mount for the thermistor head which is capable of use throughout the entire useful frequency range of the waveguides.

A further object of the invention is to devise a mount for a thermistor bead in which the bead impedance is mainly resistive in nature and is substantially constant throughout the useful frequency range of the waveguide.

For attainment of this specific object, the mount is de-.

signed so that the terminal leads of the bead are relatively short. Specifically, the head is mounted in a section of ridged waveguide, within the space between the edge of the ridge and the opposite broad wall of the waveguide.

The thermistor mount described herein is especially useful for the measurement of microwave energy transmitted in rectangular waveguides up to the K band frequencies, that is, frequencies from about 8 to about 12.5 kilomegacycles per second.

One embodiment of the invention is illustrated in the accompanying drawing in which Figure l is a sectional view taken along a central longitudinal plane passing through the thermistor mount, and Figure 2 is a front end view of the mount shown in Figure 1.

Within the frequency range indicated above, thermistor beads present a predominantly resistive impedance where the connecting leads are kept relatively short. By providing a mounting for the bead with short leads, and by mounting the bead in a line the characteristic impedance of which is very nearly constant over the desired frequency range, a broad band response would be obtained, provided the impedance seen by the thermistor in back of it is very high with respect to its own impedance. The ideal situation would be an open circuit in parallel with the thermistor.

Referring to the accompanying drawing, the thermistor bead shown at 1 is mounted near the rear end of a rectangular waveguide section 2 having a coupling flange 3 at the front end thereof for connection to a standard rectangular waveguide suitable for transmission of frequencies over the desired band. The rectangular opening at the flanged end of the section 2 is of the same size as the opening in the standard waveguide to which the section is coupled.

Near the rear end of section 2 and on one broad wall thereof is mounted a coaxial cable coupling 4 having an inner conductor 5 terminating in a probe 6 which extends through an opening in the top wall of the section 2 and forms a terminal for one of the connecting leads of bead 1. A conducting head or washer 6a is carried by the center conductor 56 on the outside of section 2 and extends radially beyond the edge of the probe hole and is insulated from the top wall of section ice 2 by a washer 6b which may be located in a recess formed in the upper face of the top wall. The bead 6a by-passes microwave energy directly to the waveguide wall but is insulated against the flow of direct current. The cable coupling 4 provides the necessary connection for supplying a direct current bias to the head 1 and for connecting the bead in a suitable indicator circuit.

Mounted within section 2 along the center of the bottom wall of the section is a conducting plate 7 which forms a ridge within the section. As shown in Figure 1, the ridge 7 is formed in stepped longitudinal sections beginning from a point just inside of the open. end of the section and extendingthrough to the rear end which is closed by metal plate 8. In front of the bead 1, the plate 7 is formed of five sections 7a, 7b, 7c, 7d and7e of different heights, and the lower terminal lead for head 1 is connected with the rear end of the section 7e. The portion 7; of the ridge 7 to the rear of the bead 1 is of smaller height than the section 7e.

For convenience in manufacture, the ridge plate 7 is -of uniform width throughout its length and is secured for microwave energy but is insulated from the wave guide section for the application of direct current to the bead through the cable connector 4.

The dimensions for the various steps in the ridge 7 are determined according to well known design technique from the following considerations:

The cut-off frequency of the waveguide section along ridged-section 7e must be very much below that of the cut-off frequency of the rectangular guide to which the section 2 is connected. The length of section 72 is not critical.

The clearance between the top of section 7e and the upper wall of section 2 should be small to reduce'the length of the thermistor terminal leads.

The width of the ridge plate 7 should be about onefifth of the internal Width of the section 2.

The back cavity to the rear of the bead must be designed to present to the thermistor head a very high impedance.

The length of the back section 71 is chosen such that substantially an open circuit is seen by the thermistor at the center frequency of the operating band. The length of this back section is critical and should be held Within 0.002 of an inch. The length of section 7 will be very close to one-quarter of the guide wavelength.

The height of the back section 7f is selected to provide a ratio of back cavity impedance to ridged guide impedance in the neighborhood of 3.

The steps in front of the bead 1 are designed to provide a substantially constant characteristic impedance at the input end of section 2 over the frequency band. The four sections of the ridged transmission line are each a quarter wavelength long at the center frequency. The physical lengths are different due to the different guide wavelengths of each section. At each step, reflections are set up and the characteristic impedances of the vari ous sections are so chosen that the sum of all reflections interfere destructively over the entire band. While only four stepped sections have been shown, a greater number of sections will produce a better response.

The effects of the back section or cavity on the input VSWR of the thermistor mount may be reduced by providing a somewhat different connection for the upper end 2,933,705 I Patented Apr. 1 9, 1960- newer of the bead. For example, instead of by-passing the microwave energy directly to the waveguide wall through the capacity bead 6a, the upper terminal of the bead may be connected to the center conductor of the coaxial cable connector 4 through a coaxial line section forming astubline having a length equal to one-half of the wavelength at the center frequency. This stub line would be embodied in the structure of the cable connector.

1 claim:

1. A mount for a thermistor bead comprising, a waveguide section of rectangular form, coupling means at one end of said section and means closing the other end thereof, a ridge plate located within said section adjacent the closed end thereof and arranged along the center of one broad wall of said section and having a free edge arranged in parallel spaced relation with the opposite broad wall of said section, a thermistor bead mounted Within the gap between said ridge plate and said opposite broad wall at a point spaced from the closed end of said section, one terminal of said bead being connected directly to the upper edge of said ridge plate, and the other terminal being connected by an insulated connection to the outside of said section through an opening in said opposite broad wall, means preventing the loss of wave'energy through said opening, and matching means interposed betweenthe open end of said section and said head for suppressing the effects of reflection from said ridge plate at the open end of said section.

2. A thermistor bead mount according to claim 1, whereinsa'id matching means comprises linear sections of said ridge plate of progressively decreasing hei-ght.

' 3. A mount for a thermistor bead comprising, a waveguide section of rectangular form, coupling means at the input end of said section, a central ridge plate located within said section at a point spaced from the input end thereof'and having a free edge arranged in parallel spaced relation with the one broad wall of said section, a thermistor bead mounted within the gap between the free edge of said ridge plate and said broad wall at a point spaced from the open end of said section, one terminal of said bead being connected directly to the upper edge of said ridge plate, and the other terminal being connected by an insulated connection to the outside of said section through an opening in said broad wall, means preventing the loss of Wave energy through said opening, matching means interposed between the open end of said section and said head for suppressing the effects of reflection from said ridge plate at the open end of said section, and terminating means for said waveguide section presenting substantially open-circuit impedance to said head.

i. A mountrfor a thermistor bead comprising, a Waveguide section of rectangular form, coupling means at the input end of said section and an end plate closing the other end, a ridge plate located within said section and extending from the center of one broad wall thereof towards the opposite broad wall, said ridge plate extending from a point adjacent the open end of said section to the end plate thereof and being formed of linear sections of increasing width towards the closed end of said section, the rear end section of said ridge plate being of reduced Width with respect to the adjacent intermediate section and providing a step in said ridge plate spaced from said end plate a distance of substantially onequarter of the guide wavelength, a thermistor bead mounted within the gap between said ridge plate and said opposite broad wall at the location of said step, one terminal of said head being connected directly to said ridge plate at said step, and the other terminal being connected by an insulated connection to the outside of said section through an opening in said opposite broad wall, and means preventing the loss of wave enengy through said opening.

References Cited in the file of this patent UNITED STATES PATENTS 2,438,915 Hansen Apr. 6, 1948 2,576,344 Howard Nov. 27, 1951 2,633,493 Cohn Mar. 31, 1953 FOREIGN PATENTS 650,614 Great Britain Feb. 28, 1951 

